The present invention relates to getter systems comprising a phase active in gas sorption inserted in a porous material distributed in a low permeability means.
Getter materials and systems are widely used in industry in all applications where it is necessary to maintain a vacuum, to control the composition of the gaseous atmosphere through the sorption of traces of undesired gases or to protect devices sensitive to particular gaseous contaminants.
Getter materials widely used for such purposes are porous materials, such as active carbons, particularly useful for the sorption of organic substances, or zeolites, silica or alumina, useful for the sorption of gaseous molecules of small size. Another class of particularly interesting compounds is comprised of anhydrous chemical desiccants, specific for moisture sorption, such as the oxides of alkaline-earth metals, or some hygroscopic salts such as chlorides (e.g., calcium chloride, CaCl2), perchlorates (e.g., magnesium perchlorate, Mg(ClO4)2), or sulphates (e.g., calcium sulphate, CaSO4).
One problem common to many of these materials is that they are generally in the form of powders without cohesion sufficient to form compact bodies. This is particularly true in the case of desiccants after moisture sorption. This is a relevant problem, as in almost all foreseen industrial applications the absence of free particles is required.
The problem is in some cases tackled by inserting the getter materials inside permeable containers (e.g., non-woven fabric envelopes, as shown for instance in U.S. Pat. No. 4,668,551 directed to insulating panels).
Another possible approach to the problem is to distribute the getter material inside a dispersing matrix, capable of retaining the getter particles in a fixed location while letting the gases pass towards the getter itself. Examples of this second solution are set forth in numerous documents. Japanese patent application publication JP 60-132274 discloses desiccant materials dispersed in a silicone matrix; U.S. Pat. No. 3,704,806 discloses desiccant compositions comprising zeolites dispersed inside a matrix formed of a thermosetting polymer, such as the epoxy resins; U.S. Pat. No. 4,081,397 discloses a desiccant system comprising particles of an oxide of an alkaline-earth metal dispersed in an elastomeric polymer; U.S. Pat. No. 5,304,419 discloses desiccant compositions comprising a desiccant material dispersed in a matrix that can be formed of silicone, polyurethanes or similar polymers; U.S. Pat. No. 5,591,379 discloses desiccant compositions comprising a desiccant selected from zeolites, alumina, silica gel, alkaline-earth metal oxides, and alkaline metal carbonates, dispersed in a matrix of porous glass or ceramic; U.S. Pat. No. 6,226,890 B1 discloses desiccant systems wherein a desiccant material (e.g., an alkaline-earth metal oxide) is dispersed in a polymer, such as silicones, epoxides, polyamides, polymethylmethacrylates or others, which in the patent is said to have the property of not reducing or even increasing the sorption speed of water by the desiccant material; U.S. Pat. No. 6,819,042 B2 discloses desiccant systems comprised of a desiccant material dispersed in a resin, e.g., a polyethylene, polypropylene, polybutadiene, or polyisoprene resin; finally, U.S. Pat. No. 6,833,668 B1 discloses a system to damp the impact of moisture on the sensitive components of organic light emitting displays (OLEDs), which is based on an impermeable resin barrier containing a desiccant powder, wherein the barrier function of such a system is pointed out by use as a means for sealing OLED cavities.
One limit that is common to many of the systems disclosed in these patents, whether based on dispersing matrices permeable to gases or having poor permeability, is that, due to the reaction with the gas to be sorbed, the getter material generally undergoes structural and morphological modifications, e.g. swellings, which, particularly in the case of desiccants, can be considerable. The presence of a matrix surrounding the particle of getter material can hinder these morphological modifications and inhibit or delay the gas sorption reactions.
In addition, some industrial applications may pose other requirements to getter systems. For instance, OLEDs of the latest generation require a getter system that is transparent and has constant optical properties throughout the whole life of the device, that is, soon after manufacture (when the getter material has not yet sorbed moisture, except for minimum amounts), near the end of the life of the device (when the getter device has already sorbed relatively large amounts of moisture, even up to saturation of the system) and also at intermediate steps of the OLED life, that is when the various getter particles dispersed in the matrix have sorbed different amounts of moisture. The different levels of moisture absorbed by getter particles during the OLED life can change optical properties of the system, such as its light transmission or refractive index, thus impairing the quality of the display. The problem is discussed, for example, in U.S. Pat. No. 6,465,953 disclosing a getter system for OLED, comprised of getter particles in a transparent matrix, wherein the particles have sufficiently small size not to interact with the luminous radiation. Given the importance of this application, in order to illustrate the uses of the getter systems of the invention, reference will be particularly made to the use in OLEDs, but the getter systems of the invention are of a general use and may be also used in other applications.